A communication system and an optical transmitter with the features recited in the preamble are known from J. J. O'Reilly, "RACE R2005: microwave optical duplex antenna link", IEE Proceedings-J, Vol. 140, No. 6, December 1993, pages 385-391. The optical transmitter shown in FIG. 2 of that article forms part of an optical communication system illustrated in FIG. 1, wherein besides an optical receiver including an optical-to-electrical transducer, an antenna is provided at the receiving end. Between this antenna and a remote radio station (transposer), radio signals with a frequency of, e.g., 60 GHz are transmitted; the wavelength of these radio signals thus lies in the range of a few millimeters. The optical transmitter (dual frequency optical source) shown in FIG. 2 of that article comprises a DFB laser, an optical modulator, a signal source, and an optical filter. The light emitted by the DFB laser is a continuous-wave signal and has an optical frequency .upsilon..sub..degree. (carrier frequency). This light is fed into one input of the optical modulator, which is controlled by a control voltage (cw-drive signal V(t)) from the signal source. The control voltage V(t) is composed of a bias voltage and a sinusoidal voltage of fixed frequency .omega.. The control voltage V(t) as a function of time is given in Equation (1) of the article, and the electric field strength as a function of time which results at the output of the optical modulator is given in Equation (3). The emerging light has a frequency spectrum with two distinct frequency components (spectral lines) which are spaced by the fixed frequency .omega. of the control voltage V(t) either side of the suppressed optical carrier frequency .upsilon..sub..degree. , i.e., the two frequency components are spaced 2.omega. apart.
The optical filter following the output of the first optical modulator separates the two frequency components. It therefore has two outputs O/P1 and O/P2. To permit optimum separation of the two frequency components, the optical transmitter has a control unit which controls the DFB laser such that the optical frequency .upsilon..sub..degree. is always tuned to the optical filter.
The above-described optical transmitter thus generates two optical components which are not modulated by an intelligence signal. To enable such an optical transmitter to transmit light modulated by an intelligence signal, one (a first) output of the optical filter is connected to an optical modulator, and the other (second) output to a coupler. The optical modulator modulates the optical component emerging at the first output of the optical filter with an intelligence signal. This modulation is an external modulation. The frequency spectrum of the modulated optical component thus has only one of the two frequency components. This modulated optical component, which appears at the output of the optical modulator, and the optical component appearing at the second output of the optical filter are coupled into an optical fiber through the coupler.
Thus, the light propagating in the optical fiber is composed of two optical components: a modulated optical component with the first frequency and an unmodulated optical component with the second frequency. In the optical receiver, this composite light falls on the surface of a PIN photodiode, where the two optical components mix coherently to produce the desired millimeter-wave signal.
As mentioned, the optical transmitter comprises an optical filter and a frequency controller for the DFB laser. The optical filter must meet stringent selectivity requirements. Such an optical filter is therefore expensive. Stringent requirements must also be placed on the frequency control, so that the latter also involves high complexity, since the optical frequency .upsilon..sub..degree. of the DFB laser and the optical filter must be mutually stabilized. This optical transmitter adds significantly to the cost and complexity of the entire communications system.